The present invention relates to systems and methods for moving a load toward a setpoint and, more particularly, relates to systems, controllers and methods for moving a load toward a setpoint in an energy balanced manner.
In many continuous processes, feedback controllers are used to generate an output that causes some corrective effort to be applied to the processes. In this regard, a feedback controller is capable of driving a measurable process variable toward a desired value, sometimes referred to as the setpoint. Generally, the controller uses an actuator, such as a motor, to affect the process and a sensor to measure the results of the process. Typically, a feedback controller determines its output by observing the error between the setpoint and a measurement of the process variable. In this regard, errors can enter into the process in any one of a number of manners, such as by an operator intentionally changing the setpoint or by a process load changing the process variable accidentally.
One type of feedback controller, a proportional-integral-derivative (PID) controller, determines the current value of the error in the system, the integral of the error over a recent time interval, and the current derivative of the error. In this regard, a PID controller determines not only how much of a correction to apply, but for how long to apply the correction. Typically, in systems including a PID controller, if the current error is large or the error has been sustained for a lengthy period of time or the error is changing rapidly, the controller will attempt to make a large correction by generating a large output. In contrast, if the process variable has matched the setpoint for a lengthy period of time, the controller will not attempt to make a large correction.
Whereas PID controllers are adequate in controlling many processes, PID controllers have drawbacks in some systems. For example, in some systems the actuator acts upon a load that generates a relatively large amount of frictional forces that oppose the actuator acting upon the load. In this regard, to control the actuator to drive the load, a traditional PID controller, that may operate with a 2-3 Hz bandwidth, would be required to overcome the static torque in the load. Thus, the traditional PID controller would be required to integrate to a high output current value and then quickly reduce the output current value to generate a running actuator torque equal to approximately half the maximum actuator torque. Then, the traditional PID controller would be required to stop the load at the setpoint.
Because the bandwidth of such a closed-loop system is often too low to capture the dramatic changes in friction seen in the many systems, a traditional PID controller generally cannot start and stop the actuator at the desired location. Also, due to the fact that the minimum current required to drive the load equals approximately half the maximum the actuator can provide, a traditional PID controller generally controls such systems with less than a desirable efficiency. Such low efficiency, in turn, generally causes significant heating of the actuator and the additional power loss generally limits the operational life of the system.
In light of the foregoing background, various embodiments of the present invention provides an improved system, controller and method for moving a load toward a setpoint. The system, controller and method move the load in an energy balanced manner where the amount of energy transferred to the load to move the load toward the setpoint equals the amount of energy lost by the load due to a frictional torque generated by the load. As such, the system, controller and method can move the load to the setpoint with one or more input pulses of short duration, as opposed to requiring a continuous input to the actuator the entire time the actuator drives the load to the setpoint. Also, because the system, controller and method move the load with short input pulses, the total power loss in the actuator can be reduced a significant amount as compared to traditional controllers.
According to one embodiment, a system for moving a load toward a setpoint includes an actuator connected to the load and a controller capable of controlling the operation of the actuator. In this regard, the actuator is capable of generating input energy that is transferred to the load to thereby move the load toward the setpoint, and the controller is capable of controlling the operation of the actuator to move the load toward the setpoint. To control the actuator, the controller is capable of determining a command distance based upon the setpoint and a current position of the load. Based upon the command distance, then, the controller can determine a pulse duration. With the pulse duration, the controller is additionally capable of applying an input pulse to the actuator for the pulse duration such that the actuator generates the input energy. Advantageously, the input pulse is applied such that the input energy equals an amount of energy lost by the load in moving the load toward the setpoint.
The system can include a drive element electrically connected between the controller and the actuator. The drive element can then receive a control signal from the controller and thereafter transmit the input pulse to the actuator based upon the control signal. Additionally, the system can include a sensor connected to the actuator and/or the load to thereby measure a current position of the load after the controller applies the input pulse to the actuator. As such, the controller can repeatedly determine the command distance, determine the pulse duration and apply the input pulse, and the sensor can repeatedly measure the actual position, until the actual position differs from the setpoint by less than a predetermined threshold.
As stated, the controller can determine the pulse duration based upon the command distance. Also, the controller can determine the pulse duration based upon a controller gain. In this regard, the controller is capable of determining the pulse duration T according to the following:
T=Kcxc3x97{square root over (xcex8cmd)}, 
where Kc represents the controller gain and xcex8cmd represents the command distance. In addition to determining the command distance, the controller can also determine the controller gain, such as based upon a moment of inertia of the load, a controller torque and a frictional torque. In other words, the controller can determine the controller gain Kc according to the following:             K      c        =                            2          xc3x97          I                                      τ            c                    xc3x97                      (                          1              +                                                τ                  c                                                  τ                  f                                                      )                                ,
where I represents the moment of inertia of the load, xcfx84c represents the controller torque and xcfx84f represents the frictional torque.
According to an embodiment that includes the sensor, the sensor can measure the current position of the load after the controller applies the input pulse to the actuator. And as such, when the current position of the load differs from the setpoint by more than a predetermined threshold, the controller can adjust the controller gain based upon the current position of the load. For example, the controller is capable of decreasing the controller gain by a predetermined amount when the current position of the load is greater than the setpoint. Also, for example, the controller is capable of increasing the controller gain by a predetermined amount when the current position of the load after moving the load is less than the setpoint. A method of moving a load to a setpoint is also provided.